1. Field of the Invention
The present invention relates generally to bits for drilling subterranean formations. More specifically, the invention relates to multiple nozzle rotary drag bits employing variations in nozzle size and orientation to apportion hydraulic flow volume on the bit face in relationship to formation cuttings volume generated by groups of cutters on the bit, as well as to bits employing junk slots with cross-sectional areas apportioned in relationship to cuttings generated by groups of cutters with which the junk slots are respectively associated, such features providing enhanced formation cuttings clearance from the bit face, through the junk slots, and into the well bore annulus above the bit.
2. State of the Art
Design of rotary drag bits employing superabrasive cutters, usually in the form of so-called "polycrystalline diamond compacts", or "PDCs," has reached a high degree of sophistication over the last several decades. Marked increases in rate of penetration (ROP) have been achieved. However, the inability of state-of-the-art rotary drill bits to clear formation cuttings at a rate commensurate with the bits' ability to generate such cuttings has proven to be a troublesome limitation to further increases in ROP.
Various designs and approaches have been employed in the art to facilitate cuttings removal from the bit, and thus facilitate increases in ROP. However, such designs and approaches have generally involved features which are not readily employable in bits of a variety of sizes and configurations, and many are limited to very specific configurations. Moreover, the prior art approaches have failed to consider and appreciate the tendency of poor cuttings clearance from a single blade of a multi-bladed bit to hinder ROP.
One prior art approach to cuttings removal from the bit involving a specialized bit design is disclosed in U.S. Pat. No. 5,417,296, wherein nozzles for supplying drilling fluid are placed both near the center of the bit and near the gage. An outer nozzle associated with one blade and fluid course on the bit face is oriented so as to provide a significant fluid flow component directed inwardly toward the centerline of the bit to augment the outward flow from an inner nozzle associated with another blade and fluid course through communication between the adjacent inner ends of the two fluid courses. Such an arrangement, in theory, enhances formation cuttings clearance, but it has been reported that this is not the case in practice. Specifically, cuttings from the blade with which the outer nozzle is associated are carried inwardly to a constriction between blades, causing clogging of the fluid course fronting that blade and consequent balling of the bit.
Accordingly, the art is, to date, devoid of enhancements to rotary drag bit design in terms of formation cuttings clearance readily applicable to improve the performance in terms of ROP of otherwise conventional bits.